This invention relates to transmission wires and means to equip them with effective electromagnetic shielding jackets.
Most elementary treatments of transmission line theory, relating to such transmission lines as coaxial and balanced shielded lines, assume the use of an outer conductor (shield) which is continuous, homogenous, and of sufficient thickness to assure no electrical current flows on the outer surface of the outer conductor as the result of electrical currents and magnetic fields within the transmission line.
In fact, none of these assumptions are strictly true and, in many applications, significant radiation emits from shielded transmission lines, e.g. because of insufficient thickness or gaps in the outer conductor. In general, lines that leak electromagnetic energy also have a tendency to pick up interfering signals. These two effects together tend to contribute to what has become recognized as a growing electromagnetic interference (EMI) problem.
Desirable solutions to these problems should provide improved shielding while maintaining flexibility and, desirably, very little increase in diameter of the transmission line or in cost of the line. Double-shielded lines do not meet these criteria well, albeit they markedly reduce emissions and find important use in many applications which can tolerate the increased diameter and cost together with the decrease in flexibility.
Thus, the shielded products available on the market tend to be cables with woven wire shields, aluminum foil shields or those embodying electroconductive tubing or conduits. Such products are expensive, or lack flexibility, or have larger diameters than would be desirable.
Other workers in the prior art have paid general attention to the problem of particular electromagnetic shielding applications by adding conductive powders to organic polymers to form electrically-conductive composites. Such materials are disclosed in U.S. Pat. No. 4,011,360 (noble metal in siloxane polymer). Non-conductive composites using metal powders generally to reflect microwave radiation have also been proposed (U.S. Pat. No. 2,951,247). Indeed, a great deal of art has been proposed for providing metal-filled composites for use in such products as microwave gaskets and the like (U.S. Pat. No. 3,746,530 discloses some such products). It should be noted that at about the microwave oven radiation frequencies, ferromagnetic powders lose their ferromagnetic character, although they do remain electroconductive.
Nevertheless, it has remained a problem to provide a suitable shielded transmission line which is at once low cost, flexible, and yet excellently shielded.
It is to be noted that the above statement of the prior art necessarily has been prepared with a hindsight view of the invention. Thus, this statement should not be construed as evidencing the state of the art as it may have appeared to one skilled in the art before the invention was made.